Darrell Brooks, 39, wept as he was arraigned in court with five counts of intentional homicide. Prosecutors said further charges would be filed.
Police say Brooks drove in a "zig-zag pattern" to hit as many people as possible at the Waukesha parade.
Bail was set at $5m (£3.7m) due to his extensive criminal background.
Three dancing grannies among Wisconsin victims
The boy killed was eight-year-old Jackson Sparks, who was hit alongside his 12-year-old brother, Tucker.
He had been marching with his baseball team when he was struck. A GoFundMe page said Jackson underwent brain surgery on Sunday and needed "a miracle".
His brother, who suffered a fractured skull, is expected by family members to recover. Tucker was among 62 people injured.
Police said 18 of the injured were children. Several victims remained in critical care in hospital.
During Tuesday's court appearance, Judge Kevin Costello said the $5m bail was due to the suspect's "extraordinary" criminal history and because he had fled previous charges in Wisconsin and Nevada.
"I have not seen anything like this in my very long career," Judge Costello said.
The other victims have been named as Virginia Sorenson, 79; LeAnna Owen, 71; Jane Kulich, 52; Wilhelm Hospel, 81; and Tamara Durand, 52.
A medical examiner has concluded they died of "multiple blunt force injuries", according to a criminal complaint released on Tuesday.
According to the complaint, Brooks ignored officers' attempts to make him leave the parade route before accelerating his vehicle into the crowd.
One officer said the driver looked like he "had no emotion on his face".
Police said the red Ford Escape SUV "appeared to be intentionally moving side to side" and witnesses felt this was "a direct intent to hit as many parade participants".
One officer shot three rounds into the SUV, but failed to stop it. Brooks was not hit by the bullets, police have said.
The Waukesha parade route
line
During Tuesday's court appearance, Brooks wore an anti-suicide smock and cried as the judge informed him that each crime carries a sentence of life in prison.
Police have said Brooks was leaving the scene of a domestic dispute minutes earlier when he drove into the parade route.
Two days before the parade attack, Brooks posted bail after being charged with running over the mother of his child in a domestic dispute earlier this month.
Milwaukee County District Attorney John Chisholm said he is now reviewing Brooks' bail of $1,000 to determine if it was set "inappropriately" low.
Mr Chisholm, a Democrat, has long advocated for lower cash bail to make it easier for criminal defendants to be released from jail pending trial.
Darrell Brooks appears in Waukesha County Court
IMAGE SOURCE,REUTERS
At the time Brooks posted bail, he was already on bail awaiting trial for allegedly firing a gun at his nephew and another person during a dispute.
Records show that he has been charged with 10 crimes since 1997.
A video has also emerged of Brooks about 20 minutes after the Waukesha parade incident asking a homeowner to come inside and use his phone.
The resident made him a sandwich and let him borrow a jacket, not knowing of the parade attack that had happened shortly beforehand.
Police arrived after the neighbour became uncomfortable and asked Brooks to leave, according to NBC News.
Witnesses have told BBC News the attack has horrified the community of 72,000 people located to the west of Milwaukee.
Tailyr Medrano, a Waukesha resident who was attending the parade with her one-year-old son, said at a vigil for victims on Monday night that she was "feet" away from getting hit.
"I watched people going down right in front of me and people fly off the hood of the car. Just images that I will never forget," she said.
Darrell Brooks, 39, wept as he was arraigned in court with five counts of intentional homicide. Prosecutors said further charges would be filed.
Police say Brooks drove in a "zig-zag pattern" to hit as many people as possible at the Waukesha parade.
Bail was set at $5m (£3.7m) due to his extensive criminal background.
Three dancing grannies among Wisconsin victims
The boy killed was eight-year-old Jackson Sparks, who was hit alongside his 12-year-old brother, Tucker.
He had been marching with his baseball team when he was struck. A GoFundMe page said Jackson underwent brain surgery on Sunday and needed "a miracle".
His brother, who suffered a fractured skull, is expected by family members to recover. Tucker was among 62 people injured.
Police said 18 of the injured were children. Several victims remained in critical care in hospital.
During Tuesday's court appearance, Judge Kevin Costello said the $5m bail was due to the suspect's "extraordinary" criminal history and because he had fled previous charges in Wisconsin and Nevada.
"I have not seen anything like this in my very long career," Judge Costello said.
The other victims have been named as Virginia Sorenson, 79; LeAnna Owen, 71; Jane Kulich, 52; Wilhelm Hospel, 81; and Tamara Durand, 52.
A medical examiner has concluded they died of "multiple blunt force injuries", according to a criminal complaint released on Tuesday.
According to the complaint, Brooks ignored officers' attempts to make him leave the parade route before accelerating his vehicle into the crowd.
One officer said the driver looked like he "had no emotion on his face".
Police said the red Ford Escape SUV "appeared to be intentionally moving side to side" and witnesses felt this was "a direct intent to hit as many parade participants".
One officer shot three rounds into the SUV, but failed to stop it. Brooks was not hit by the bullets, police have said.
The Waukesha parade route
line
During Tuesday's court appearance, Brooks wore an anti-suicide smock and cried as the judge informed him that each crime carries a sentence of life in prison.
Police have said Brooks was leaving the scene of a domestic dispute minutes earlier when he drove into the parade route.
Two days before the parade attack, Brooks posted bail after being charged with running over the mother of his child in a domestic dispute earlier this month.
Milwaukee County District Attorney John Chisholm said he is now reviewing Brooks' bail of $1,000 to determine if it was set "inappropriately" low.
Mr Chisholm, a Democrat, has long advocated for lower cash bail to make it easier for criminal defendants to be released from jail pending trial.
Darrell Brooks appears in Waukesha County Court
IMAGE SOURCE,REUTERS
At the time Brooks posted bail, he was already on bail awaiting trial for allegedly firing a gun at his nephew and another person during a dispute.
Records show that he has been charged with 10 crimes since 1997.
A video has also emerged of Brooks about 20 minutes after the Waukesha parade incident asking a homeowner to come inside and use his phone.
The resident made him a sandwich and let him borrow a jacket, not knowing of the parade attack that had happened shortly beforehand.
Police arrived after the neighbour became uncomfortable and asked Brooks to leave, according to NBC News.
Witnesses have told BBC News the attack has horrified the community of 72,000 people located to the west of Milwaukee.
Tailyr Medrano, a Waukesha resident who was attending the parade with her one-year-old son, said at a vigil for victims on Monday night that she was "feet" away from getting hit.
"I watched people going down right in front of me and people fly off the hood of the car. Just images that I will never forget," she said.
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